We all know that its the chemical enzymes that complete out digestion, and there is is a specific enzyme for each molecule to be absorbed by.But why can't one type of digestive enzyme hydrolyses another? e.g. a sucrase can only hydrolyse sucrose into glucose and and fructose but it can't hydrolyse lactose why?

Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream, so in the absence of lactase, lactose present in ingested dairy products remains uncleaved and passes intact into the colon. The operons of enteric bacteria quickly switch over to lactose metabolism, and the resulting in-vivo fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane).

Enzymes are extremely specific for the structures (shapes) of the substrates (materials) upon which they will act.

This is because the substrate molecule has to fit into a chemically-specialised cleft or groove on the surface of the enzyme called the active site. Here, chemical interactions occur between atomic groups in the enzyme and specific parts of the substrate molecule. This allows the enzyme to chemically modify the substrate by permitting reactions to occur at much lower energies than would be required in the absence of the enzyme.

But if the substrate molecule is not the correct one then it will not interact with the enzyme's active site in the correct way; the chemical groups of the substrate will not be compatible with the chemical groups of the enzyme and so no reaction, or at best only a very slow reaction, can occur.

Some people refer to this as the "lock and key" hypothesis whereby only the appropriate key (substrate) can fit into a given lock (enzyme). So a sugar of one type is the wrong shape to be chemically acted upon by an enzyme that normally modifies a different sugar.

In the case of lactose, this is a disaccharide (double sugar) containing a molecule of galactose linked to a molecule of glucose; it comprises about 5% of the sugars in a cow's milk. But before it can be usefully employed within the body it has to be broken down to its two consitutent sugars (glucose and galactose), both of which can be fed into energy-yielding metabolic pathways.

The view of many is that this break-down occurs within the lumen (tube) of the small intestine; but it's more likely that the enterocytes (intestinal lining cells) do this chemical modification on the "brush border" - the villi that increase surface area - as they absorb the products of digestion.

This was shown by Dahlqvist and Borgstrom, 1961:

"Both the low glycosidase activity measured and the low degree of hydrolysis of disaccharides in the intestinal contents during the digestion and absorption of the test meal indicate that the hydrolysis of disaccharides in the human intestine does not occur in the intestinal lumen, and that disaccharides are absorbed as such. It is well established that disaccharides infused into the systemic circulation are essentially quantitatively excreted by the kidneys (Verzar & McDougall, 1936). This observation, when added to the high glycosidase activity of mammalian intestinal mucosa (Dahlqvist, 1961), seems to indicate that absorbed disaccharides are hydrolysed by intracellular mucosal enzymes and the monosaccharides pass into the portal system."

But if the brush border lacks lactase activity then the disaccharide molecules cannot be picked up and hence they pass into the colon where they provoke a feeding frenzy among the local flora, which are unused to such rich pickings normally. The result is disturbed microflora, altered bowel habit and the symptoms of lactose intolerance, including diarrhoea secondary to the osmotic effects of the sugars.

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